The present invention relates to a tubular circuit connector or, more particularly, to a rubber-based tubular circuit connector used for electric connection of circuits, for example, between a liquid crystal display unit and a circuit board for driving the same, between two display units or between two circuit boards as well as to a unique method for the preparation thereof.
As is well known, a variety of modern electric and electronic instruments are manufactured by assembling a plurality of working units, e.g., liquid crystal display modules, and circuit boards to provide an electric circuit for driving the working unit. The electric connection between the electrode terminals of the working units and the circuit boards is established by using an electric circuit connector, referred to simply as a connector hereinafter. While various types of connectors are known and employed in the prior art, the most widely employed connector is a rubber-based connector of which the reliability of electric contacting between electrode terminals is ensured by utilizing the elastic resilience of a rubber-made member. For example, the so-called "zebra"-type rubber-based connector is an integral elongated body having a stratified structure consisting of a lengthwise alternate stratification of a multiplicity of layers of an electrically insulating rubber and a multiplicity of layers of an electroconductive rubber, which is a composite rubber compounded with an electroconductive powder such as a conductive carbon black and silver powder, to exhibit a black-and-white striped appearance.
It is usual in the above described "zebra" connectors that the composite rubber forming the conductive layers has a relatively high volume resistivity so that the connectors of this type are not quite satisfactory for electric connection between electrode terminals where a low electric resistance is essential as in the connection of color liquid crystal display modules and monochrome liquid crystal display modules of 18 or more gradations because of the possible variation in the performance of the liquid crystal display as being affected by the contact resistance unless the contact resistant is controlled to be negligibly small. The "zebra" connectors are also not suitable for electric connection when the electric current passing each of the conductive layers is large to exceed, for example, 10 mA as in the connection of plasma display modules because of the temperature elevation due to the heat generated therein as a consequence of the high resistance.
Furthermore, the rubber layers constituting the "zebra" connectors have a relatively high rubber hardness so that reliable electric connection can hardly be obtained between the electrode terminal and the conductive layer of the connector unless the contacting pressure therebetween is unduly increased to such an extent that the circuit board for connection is under a risk of warping or distortion by the large contacting pressure which may lead to a decrease in the stability of electric connection with the connector. This problem of course can be solved by increasing the thickness of the substrate board of the circuit board to withstand the large contacting pressure. This means, howqever, can not always be employed because an increased thickness of the circuit board requires on the other hand a decrease in the thickness of the working unit such as the liquid crystal display modules, for example, in mobile telephones which must be very compact in volume and light in weight. When the electroconductive rubber forming the conductive layers of the "zebra" connector is a composite rubber compounded with silver particles, in addition, the phenomenon of electromigration of silver atoms sometimes takes place between the electrodes to cause deposition of silver metal on the electrode surface resulting in a decrease in the reliability of electric connection through the connector.